Thumbnail generation fails on video files

Why don't I have thumbnails for my video files?

Why thumbnail generation stops at the first video file I have ?

First, check if it's the same in konqueror.

Digikam uses KDE to generate thumbnails and KDE rely on aRts to do that.
So, you should check your aRts configuration. ARts can use xine plugin for video files, so try if xine is able to play your video files. You may have to use additional codecs (eg libxine-extracodecs on debian/ubuntu if you already have libarts1-xine) for that.
Try also to play your video files using xine (if libarts1-xine is installed).

Remember that if it doesn't work in konqueror, it won't work in digikam...